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S.A. McFadden, R. Atkinson, N. Bilton, M.H. C. Howlett, M. Hunter; The Effect of Hypothyroidism on Eye Growth and Emmetropisation in the Chick . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1140.
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The rapid ocular growth seen during development is normally accompanied by coordinated changes in optical power so that emmetropisation is achieved and maintained thereafter. Emmetropisation is disrupted in animals raised with spectacles, with ocular growth rates changing to compensate for the imposed defocus. This compensatory mechanism is sensitive to the sign of defocus, with growth rates accelerated or inhibited after negative or positive lens–wear respectively. Little is understood about the biochemical pathways underlying the moderation of ocular growth rates, but one factor which changes in concert with the sign of ocular change is retinoic acid (RA). Furthermore, when RA is fed to chicks or guineapigs, ocular elongation rates triple, yet the eye still maintains emmetropia. Thus emmetropisation is not affected under conditions in which ocular elongation is abnormally accelerated. In the current study, we studied ocular growth in chicks with induced hypothyroidism which causes a decline in retinoid receptors and a general inhibition in growth.
Chicks were either raised normally (n=10) or were fed a diet of propylthiouracil (PTU) and methimazole (MMI) to reduce levels of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3) (n=10) for a period of 8 weeks. Body weights and serum levels of T4 and T3 were measured repeatedly, and at the end of this period, refractive error and high frequency ultrasound were done under halothane anaesthesia.
The PTU/MMI diet continuously reduced the serum levels of both thyroid hormones. At 8 weeks, T4 levels in the birds fed PTU/MMI compared to normal were 14.2 Vs 10.9 pmol/L (p<0.000) and T3 levels were 4.8 Vs 2.2 pmol/L (p<0.001) respectively. The weight of birds on the PTU/MMI diet at 8 weeks of age was half that of normal birds (471 Vs 980 gms, p<0.001). Their axial lengths were 1.65mm less (14.2 Vs. 12.7 mm, p<0.001), primarily due to a reduction in the growth of the vitreous chamber (depth of 7.8 Vs 6.6 mm, p<0.001). Despite their considerably smaller eyes, the refractive error in the birds fed the thyroxine deficient diet was the same as in normal birds (0.6 Vs 0.5 D respectively).
Despite the fact that ocular growth rates were considerably reduced, the eye was still able to compensate so as to maintain emmetropia. It remains to be seen whether under such growth inhibited conditions, the eye requires vision to achieve this balance.
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